1. Field of the Invention
The present invention relates generally to an electrical contact material. Specifically, the present invention relates to an electrical contact material utilized in a variety of breakers and switches, where electric current varies intermittently.
2. Description of the Background Art
Generally, for electrical contact material for breakers or switches such as a vacuum interrupter, metals or alloys having characteristics of good electrical conductivity, low contact resistance, as well as being arc-proof and welding-proof are preferable. Conventionally, Cu--Cr alloys obtained by powder metallurgy techniques have been well known as such electric contact materials. Copper powder prepared by electrolytic methods, for example, and chromium powder prepared by milling are mixed then compacted under pressure. After compacting, the mixed powder is sintered to obtain desired Cu--Cr alloy. As a suitable electrical contact point, homogeneous distribution of Cr into a Cu matrix is necessary for obtaining the aforementioned characteristics. Further to say, the finer diameter of Cr particle, the better for the material.
However, particle distribution in materials of Cr prepared mechanically by milling methods becomes widely dispersed. Additionally, homogeneous fineness of Cr particle cannot be established easily. Therefore, weight variation occurs due to differing particle size, and such Cr particles cannot be homogeneously mixed with Cu powder. Therefore, Cr particles cannot be dispersed finely and homogeneously in the Cu matrix of a compacted article after sintering.
Classification of Cr particles using sieving means are effective for homogeneous distribution of fine particles, however, it causes severe degradation of yield and raises production cost.
Further milling of Cr particles using mechanical techniques is available to obtain fine particle size, but the surface of Cr particles is susceptible to the effects of oxygen in a course of mechanical processes. Therefore, oxidation of the Cr particle surfaces occurs in the process of milling and during storage, and the sinterability of the mixed powder becomes reduced.
Thus, the mean particle size of Cr compacted in an article prepared by conventional mechanical milling is limited in about 40 .mu.m. Additionally, particle distribution of Cr cannot be accomplished uniformly.
Casting methods for forming Cu--Cr alloys also cannot be adopted, as the slow cooling speed of alloy solidification allows the size of Cr particles in the Cu matrix to be increased. Therefore, uniform distribution of fine Cr particles cannot be accomplished easily, further to say, segregation is apt to occur during solidification.